N-methyl-
norsalsolinol and related
tetrahydroisoquinolines accumulate in the nigrostriatal system of the human brain and are increased in the cerebrospinal fluid of patients with
Parkinson's disease. We show here that 6,7-dihydroxylated
tetrahydroisoquinolines such as N-methyl-
norsalsolinol inhibit
tyrosine hydroxylase, the key
enzyme in
dopamine synthesis, by imitating the mechanisms of
catecholamine feedback regulation. Docked into a model of the
enzyme's active site, 6,7-dihydroxylated
tetrahydroisoquinolines were ligated directly to the
iron in the catalytic center, occupying the same position as the
catecholamine inhibitor
dopamine. In this position, the
ligands competed with the essential
tetrahydropterin cofactor for access to the active site. Electron paramagnetic resonance spectroscopy revealed that, like
dopamine, 6,7-dihydroxylated
tetrahydroisoquinolines rapidly convert the catalytic
iron to a ferric (inactive) state.
Catecholamine binding increases the thermal stability of
tyrosine hydroxylase and improves its resistance to proteolysis. We observed a similar effect after incubation with N-methyl-
norsalsolinol or
norsalsolinol. Following an initial rapid decline in
tyrosine hydroxylation, the residual activity remained stable for 5 h at 37 degrees C. Phosphorylation by
protein kinase A facilitates the release of bound
catecholamines and is the most prominent mechanism of
tyrosine hydroxylase reactivation.
Protein kinase A also fully restored
enzyme activity after incubation with N-methyl-
norsalsolinol, demonstrating that
tyrosine hydroxylase inhibition by 6,7-dihydroxylated
tetrahydroisoquinolines mimics all essential aspects of
catecholamine end-product regulation. Increased levels of N-methyl-
norsalsolinol and related
tetrahydroisoquinolines are therefore likely to accelerate
dopamine depletion in
Parkinson's disease.